Timber Parallel chord Truss Roof Bracing 3

[SamaKaka]

Hello!
I am currently designing a parallel chord timber roof truss. The hall dimensions are LXBxH 28mx15mx7m. The truss are spread along the length of the building, the first and the last truss lies directly at the timber walls. The roof covering provides no bracing. I am having a problem on finding good roof bracing against wind or seismic load. I am thinking to provide roof bracing at the top chord of the truss by wind bracing, at the bottom chord of the truss it is not possible to provide bracing due to architectural reasons. But even if I provide racing at the top chord of the truss, i still have hing connection between the truss and the top of the wall. My question is what kind of bracing should i be considering to transfer the horizontal load induced by wind or seismic action to the exterior walls? Is it even possible to get a desired roof bracing with out purlin at the top chord of the roof truss? Any tips might be helpful. refer the sketch attached

Thank you.

 

[CivilSigma]

Edit:
Sorry, please ignore my post below, I may have miss read the post.

My question is what kind of bracing should i be considering to transfer the horizontal load induced by wind or seismic action to the exterior walls?

Do you mean connection?
Bracing would be 2×4 purlins along the web trusses, or sheathing the top chords. In roof truss application, I wouldn't design truss bracing to resist the lateral loads. I would design the bracing to reduce a truss member's unsupported length. If I remember correctly, you design the purlins to resist like 10% of the internal web member force.



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You can fasten the trusses using hurrican ties. That will transfer some lateral load to the walls, and resist some uplift.

Check out Simpson Strong Tie H2A ties

https://www.strongtie.com/rooftruss...ndstraps_coldformedsteelconstruction/category

 

[SamaKaka]

@CivilSigma Thank your for the reply. Do you mean by providing hurricane ties to connect the bottom chord of the truss (in both ends) to exterior wall,could provide horizontal stiffens desired?

I think i did not make my self clear sorry about that, My question was how to achieve horizontal stiffness which act as a diaphragm, so that the flat roof timber truss will be stiff enough to transfer horizontal loads from x and y direction to exterior walls?

 

[BAretired]

Hello!
I am currently designing a parallel chord timber roof truss. The hall dimensions are LXBxH 28mx15mx7m. The truss are spread along the length of the building, the first and the last truss lies directly at the timber walls.
1. The roof covering provides no bracing. I am having a problem on finding good roof bracing against wind or seismic load. I am thinking to provide roof bracing at the top chord of the truss by wind bracing, at the bottom chord of the truss it is not possible to provide bracing due to architectural reasons.
2. But even if I provide racing at the top chord of the truss, i still have hing connection between the truss and the top of the wall. My question is what kind of bracing should i be considering to transfer the horizontal load induced by wind or seismic action to the exterior walls?
3. Is it even possible to get a desired roof bracing with out purlin at the top chord of the roof truss? Any tips might be helpful. refer the sketch attached

In response to your three questions:

1. I don't know why you have selected a roof system which provides no bracing. Normally, the roof deck constitutes a diaphragm. However, if that is the case, it will be necessary to provide horizontal trusses in the plane of the roof which are adequate to carry lateral forces to the perimeter walls.

2. If the walls extend to roof level, this is not an issue. If not, you simply need to provide an X-brace between trusses in two or more bays.

3. The sketch suggests a truss spacing of 5.6m to make up a total length of 28m. You will need to use purlins to span that distance, so roof bracing will not be a problem. If you wish to avoid purlins, you might consider joists or trusses spaced closer together. You might also consider using a deck which acts as a semi-rigid diaphragm.

BA

 

[SamaKaka]

@BAretired Thank you for the reply.

1, The reason for this type of roof system is the desire of the planner, the plan is said to be copy of an old hall which was demolished. They want to build exact same thing and the is no actual plan form the previous hall, to see how it was built.
There is a only developed plan to represents the old design, which is a bit confusing to me. But anyway Horizontal trusses as you mentioned makes more sense.
2. The walls are not extended to the roof level, they are up to the bottom of the roof truss. In that case also as you mentioned x bracing between trusses along the wall makes more sense.
3. The spacing of the trusses is 1.25 m, i did not present it correctly it in my sketch, my apology.



Thank you for your time,

 

[BAretired]

1. I don't know how you can build the exact same thing when the original building has been demolished and there are no structural drawings. And, in any case, who is going to know the difference? I suggest building the new building in approximate agreement with the old one but don't lose any sleep over minor differences.

2. Okay

3. 1.25m is not modular with 28m, but I suppose you could vary the end spans. Is 1.25m intended to match the original building?

4. The trusses appear to be unusually deep on your cross section. Do you know the depth of the original trusses?

BA

 

[SamaKaka]

@BAretired
1) The planner idea is to use more or less the same quantity of material as it had been done on the old Hall. That is why i am looking to find a safe solution to the desired plan, the Hall will be build in seismic zone 3, which is the highest zone in Germany.
3) Yeh, it was done to match the original building, the end span will vary.
4) The truss has 5 degree slope, and at lower end (one end) of the truss the vertical chord has app. 90 cm depth and at upper end (other end ) app. 2 m. The cross section of the members were b/h= 70mm/160 mm

It would have been more ideal to plan new cost effective but it is the desire of the client. Unfortunately.

 

[dhengr]

SamaKaka:
It is really hard to imagine how stupid some of these problems are (or become) without the guidance of an intelligent structural engineer giving good sound advice. Why, oh why, are we so cowed by some dumb designer who can’t tell his ass from a hole in the ground when it comes to leading/directing a project to a meaningful, practical, economical, current code complaint solution? If it generally looks right (historically right) from the outside and interior, who cares exactly what goes on btwn. the ceiling and the roof planes, etc. Of course, it has to be structurally sound and meet today’s codes, it has to carry the vertical load and the potential lateral loads. We are part of the design team, and we may be the smarter part of the team, if we weren’t so chicken to express our opinions. But then, when you show 5 truss spaces in your sketch, when there are actually 22 truss spaces, you can’t be counted in that ‘intelligent structural engineer’ group. Do you have no concept of what that difference in truss spacing has when an experienced engineer looks at that roof framing system? What you have done so far is at least careless, and maybe bordering on negligent. If we can’t express ourselves btwn. engineers, who should basically speak the same language, how on earth do we expect these things to actually be built.

 

[SamaKaka]

@dhengr Thank you for taking your time to express your opinion. You have a nice day.

 

[oldrunner]

You should design the roof as a plywood diaphragm. Mitek should have some bracing details. Look at the Truss Plate Institute for other information.

https://www.tpinst.org/.

You have a rather simple project but it can become complicated with your "architect". It's probably helpful if you have some old pictures. Most likely wind will be your controlling design. Check this out:

https://www.woodworks.org/wp-content/uploads/Part-3-Diaphragm-Design-Examples.pdf.

Pages 23 and 24 focuses on diaphragm fundamentals.